The present invention concerns the field of refined low colored sugar manufactured from a colored aqueous sugar solution.
In the beet sugar production, a beet refined granulated sugar with 20-30 ICUMSA color units (abbreviated I.C.U. hereafter) is usually crystallized from a standard feed liquor of about 3000 I.C.U. and 94% sugar purity. For example, a beet standard liquor of 2864 I.C.U. gives a sugar of 29 I.C.U.
However, a cane refined sugar with 20-40 I.C.U. is generally produced from standard feed liquor of 200 to 300 I.C.U. and 99% sugar purity.
Thus, it appears that the occlusion (adsorption/absorption) of colorants into refined sugar crystals is considerably greater for cane refined sugar production. Therefore, for production of cane refined sugar, extracted sucrose from sugar cane is first produced in the form of cane raw sugar with a color ranging from 1500 to 6000 I.C.U. in a raw sugar mill. The cane raw sugar is then further refined into final refined sugar product, with a color ranging from 20 to 50 I.C.U. in a sugar refinery.
The refining process primarily is a decolorization process using (a) mechanical separation (affination); (b) chemical color separation (carbonation or phosphatation); (c) physical separation (ion exchange resin, granulated activated carbon bone char process, etc.); and (d) final crystallization, which also transforms sucrose into soluble solid granulated sugar. Steps (a) and (b) can be classified as xe2x80x9cprimary decolorizationxe2x80x9d. Step (c) can be classified as secondary decolorization.
The above refining process is extremely cost intensive in both capital investment and operation cost, particularly in energy requirement and plant maintenance. If a carbonation or phosphatation process is used, disposal of carbonate cake or phosphate scum is increasingly an environmental problem. If a cane refined sugar with a color of 30 to 70 I.C.U. could be crystallized from a standard cane liquor of 3000 I.C.U. color, as it is the case of beet sugar production, then the entire refinery decolorization steps, e.g. affination, carbonation/phosphatation, ion exchange resin, all can be eliminated resulting in a tremendous saving in both capital, operation and disposal cost. Furthermore, if cane refined sugar of 30 to 60 I.C.U. could be crystallized from a cane raw sugar mill""s evaporated syrup with a color ranging from 5000 to 10,000 I.C.U., refined sugar could be produced in raw sugar mills. At any rate, if cane refined sugar could be crystallized from a standard liquor having a color anywhere between 500 to 10,000 I.C.U., selective refining process step could be eliminated resulting in considerable saving in refined sugar production.
The distinct difference in occlusion characteristics, thereafter termed as xe2x80x9cocclusion indexxe2x80x9d, between beet sugar and cane sugar colorants has been a subject of speculation and postulation for many years. That xe2x80x9cocclusion indexxe2x80x9d is defined as the color ratio between the color of the sugar and the color of the crystallization liquor: O.I.=100xc3x97col. Sugar / col. Syrup.
Because of the inability of the researchers in the field to adequately explain it, with some degree of technical and scientific certainty, the obvious difference in occlusion between beet and cane colorants are generally attributed to the nature of the colorants which have been studied by Lionnet, G. R. E. [(1987), Impurity transfer during A-massecuite boiling, Proc. S. African Sugar Technol. Assoc., p. 70 and Shore, M., Broughton, N.W. et al. (1984), Factors affecting white sugar color, Sugar Technology Review. 12:1-99].
Recent studies, Godshall, M. A. and Clarke, M. A., [xe2x80x9cHigh molecular weight color in refineriesxe2x80x9d, Proc. Conf Sugar Process. Research, pp. 75-95, (1988)] indicated that the high molecular weight colorants are preferentially occluded. Donovan, M. and Williams, J. C., [xe2x80x9cThe factors influencing the transfer of colour to sugar crystalsxe2x80x9d, Proc. Conf. Sugar Process. Research, pp. 31-48, (1992)], in a study on color transfer, also seem to have a similar finding. Although the findings of these studies are subject to speculation, they pointed to the need to search for the cause of preferential occlusion in crystals in order to find a solution to reduce xe2x80x9cthe occlusion indexxe2x80x9d of feed liquor colorant.
The so-called NAP process developed by APPLEXION (U.S. Pat. No. 5,554,227 and U.S. Pat. No. 5,902,409) showed that, by using a system as tight as a filtration membrane, due to the removal of high molecular weight molecules, the color of the sugar is drastically reduced with respect to the direct decolorization of the juice. With such a NAP process, a decreasing of the occlusion index is observed; moreover, approximately 40 to 50% of color and ash reduction is observed on crystalized sugar.
Carpenter, F. and Deitz, V. R. (Technical report, NBS report 7750, Bone char Research Project, Inc., NBS project 1502-20-15122, 19-69, 1962) clearly showed that adsorption of colorants by bone char is greatly diminished in the presence of an excess polyvalent anions. This indicates that comparatively small changes in the ionic composition of a liquor/syrup may cause a large change in the effluent liquor, i.e. color is not xe2x80x9cpicked-upxe2x80x9d by the bone char. In other words, an excess of polyvalent anions in a sugar solution containing colorants, will reduce the adsorption/absorption of said colorants on bone char, resulting in a poor decolorization by the bone char. However, if the colorant molecular weight is high enough, adsorption by granular carbon can be anticipated.
The present invention is based on the hypothesis that occlusion (adsorption/absorption) of colorants into sugar crystals during a crystallization process, could follow the same rules as adsorption on bone char.
This hypothesis has been thoroughly studied by the inventors.
Therefore, in one aspect, the present invention concerns a process for decreasing the occlusion of colorants in sugar crystals during the crystallization step of an aqueous sugar solution containing colorants, polyvalent cations such as Ca2+ and Mg2+ ions and possibly polyvalent anions, said process being characterized in that it comprises the step of treating said solution so as to increase the number of polyvalent anion equivalents with regard to the number of polyvalent cation equivalents.
In a further aspect, the present invention concerns a process for manufacturing crystallized sugar from an aqueous sugar solution containing colorants, polyvalent cations such as Ca2+ and Mg2+ ions and possibly polyvalent anions, said process comprising the step of submitting said solution to a crystallization procedure to obtain a crystallized sugar, the process being characterized in that, in order to decrease the occlusion of colorants in the crystals of said crystallized sugar, it further comprises the step of treating said solution so as to increase the number of polyvalent anion equivalents with regard to the number of polyvalent cation equivalents.
According to this invention, it is possible to reduce the occlusion of colorants in the sugar crystals, that is to reduce the above-defined occlusion index and, hence, to obtain less colored sugar crystals.
Advantageously, the treating step according to the present invention comprises the step of adding a source of polyvalent anions to said solution.
The treating step in the above processes may lead to a solution containing an excess of polyvalent anion equivalents with regard to the polyvalent cation equivalents.
Having in mind that a beet or cane sugar solution usually contains mainly chloride as monovalent anions and Ca2+ and Mg2+ as polyvalent cations, such an excess of polyvalent anion equivalents (EPA) for such a sugar solution may generally be expressed as follows:
EPA=TAxe2x88x92Clxe2x88x92xe2x88x92Ca2+xe2x88x92Mg2xe2x88x92
wherein TA=total anion equivalents, and
TAxe2x88x92Clxe2x88x92=polyvalent anions equivalents.
The source of polyvalent anions which may be added to the solution to be treated is preferably chosen among the materials which, when added to said solution, do not decrease the pH of the latter.
The source preferably provides polyvalent anions chosen among PO43xe2x88x92 ions, SO42xe2x88x92 ions, SO32xe2x88x92 ions, CO32xe2x88x92 ions and the combinations thereof, but the invention is not limited to such particular source of polyvalent anions.
The source of polyvalent anions is more preferably chosen in the group constituted by Na2CO3, K2CO3, Na2SO4, K2SO4, Na2SO3, Na3PO4, K3PO4 and the combinations thereof.
It is to be noted that said source of polyvalent anions is preferably added in an amount sufficient to raise the pH of said solution to 6.5-9.5, preferably 8.5. The addition of polyvalent anions serves to two objectives.
The addition of polyvalent anins serves to two objectives.
First, they precipitate out Ca2+ and Mg2+ usually contained in the solution which in turn increases the number of polyvalent anion equivalents with regard to the number of polyvalent cation equivalents.
Secondly, they increase themselves the number of polyvalent anions with regard to the number of polyvalent cation equivalents. After the addition of these polyvalent anions, the resulting precipitate, even if not visible, preferably should be removed, although in some cases, it is not necessary. The process of removal, after treatment of the sugar solution includes, but is not limited to, a conventional process filtration, a membrane filtration, a cross flow membrane filtration, floating and settling.
Therefore, according to another preferred embodiment, the above processes of the invention further comprise the step of filtering said solution, this step being advantageously carried out after said step of treating said solution and before the crystallization operation.
It is preferred that said filtering step includes a membrane filtration, such as a microfiltration, ultrafiltration or nanofiltration using an organic or mineral membrane well-known in the art.
During the processing of the sugar solution to produce crystallized sugar, the color of said solution should be kept as low as possible, by minimizing color formation. This can be achieved by addition of SO2 and/or any source of SO2 such as a metabisulfite, for example sodium metabisulfite.
Said addition, which is nothing else than a sulfitation, is carried out after the step of treating said solution and before or after the above mentioned filtration. It is to be noted that SO2 and/or the source of SO2 is preferably added in an amount sufficient to decrease the pH of the solution to 6.0-8.5, preferably 8.0; it is also essential to bring down the pH with SO2 in order to avoid excessive color formation due to high pH.
Table 1 below shows the effect of SO2 addition on color after heating (95xc2x0 C., 1 hour) for various kinds of sugar solution.
It is apparent that SO2, not only reduces the color of the sugar solution to be crystallized, by chemical reduction action, but also prevents the color formation during processing/heating. In addition, the oxidation products of SO2 are also polyvalent anions which increase the number of polyvalent anion equivalents, resulting in lower color of the crystallized sugar.
According to an embodiment of the invention, the crystallization procedure is carried out so as to obtain large crystals.
In this respect, it is to be noted that an important amount of color is present on the surface of the sugar crystals. Therefore, for a given weight of sugar crystals, the total surface area of the crystals increases when the size of the crystals decreases and, consequently the total color increases; that is the reason why it is advantageous to control the crystallization procedure in order to produce large crystals.
Furthermore, according to the present invention, it is also advantageous that the crystallization process comprises a step of washing of the formed crystals with water to remove all or part of the surface color, to further decrease the sugar color.